Americans in two dozen states from the Midwest to the Southeast and Northeast are shivering this week courtesy of a distorted polar vortex. The rush of cold air it's sending southward is the biggest visitor from the North Pole since Santa Claus. The gifts it brings, however, are chilling and generally unwelcome. Much of the United States has plunged into a deep freeze from record low temperatures.
CNN International senior meteorologist Brandon Miller answers a few pressing questions about this phenomenon.
What is a polar vortex? What distinguishes it?
The polar vortex, as it sounds, is circulation of strong, upper-level winds that normally surround the northern pole in a counterclockwise direction -- a polar low-pressure system. These winds tend to keep the bitter cold air locked in the Arctic regions of the Northern Hemisphere. It is not a single storm. On occasion, this vortex can become distorted and dip much farther south than you would normally find it, allowing cold air to spill southward.
Ice forms around rocks on the Brooklyn waterfront, across from Lower Manhattan, on Tuesday, January 7, in New York City.
A woman braves the cold in Scranton, Pennsylvania, on January 7.
Passengers wait for a train to arrive in Chicago on January 7, when temperatures were below zero.
A man clenches his fists while walking past a steam vent in New York City on January 7.
A man bundles up against the cold in New York City on January 7.
A couple walks past an advertisement in Pittsburgh during sub-zero temperatures January 7.
A woman tries to stay warm while waiting for a bus in Brooklyn, New York, on January 7.
Ice builds up along Lake Michigan at Chicago's North Avenue Beach on Monday, January 6.
With temperatures nearing 0 degrees, a cloud of steam from a manhole blows across an intersection in downtown Pittsburgh on January 6.
Men battle the bitter wind as they walk in Chicago on January 6.
Postal worker Jamie Jasmon struggles to deliver mail in the snow January 6 in Springfield, Illinois.
Vehicles drive down a snow-covered road in Burton, Michigan, on January 6.
Metra employees in Chicago walk around LaSalle Street Station on January 6.
A man warms his hand while running errands in Flint, Michigan, on January 6.
A commuter walks past warming lamps at a Chicago train station on January 6.
A woman props up her feet on her luggage January 6 after her flight home was canceled at Logan International Airport in Boston.
A woman makes her way through a snow drift in downtown St. Louis on January 6.
A man shovels the snow off his car in Indianapolis on January 6.
A woman walks to the University of Minnesota's St. Paul campus on January 6.
A man clears the sidewalk in front of his home in Carmel, Indiana, on January 6.
A blanket of fog covers Lake Michigan along the Chicago shoreline January 6.
A man braves the snow in downtown Springfield as a winter storm moves across the Midwest on Sunday, January 5.
A car is buried in snow January 5 in Zionsville, Indiana.
A city snowplow clears the street in an almost-deserted downtown Springfield on January 5.
With the Gateway Arch in the distance, a person struggles to cross the street during a snowstorm January 5 in downtown St. Louis.
Snow covers a garden gnome in St. Louis on January 5.
Anna Maksimkina of Yekaterinburg, Russia, sleeps on the floor at John F. Kennedy International Airport in New York after a Delta flight from Toronto to New York skidded off the runway into a snow bank, temporarily halting all flights January 5.
Four homeless men warm themselves on a steam grate by the Federal Trade Commission Building, blocks from the U.S. Capitol in Washington, as frigid temperatures grip the nation's capital on Saturday, January 4.
People go sledding in Prospect Park in Brooklyn, New York, on January 4.
Snow is piled high in front of a Home Depot in Boston on January 4.
Sunlight beams through the windows of a building that caught on fire Friday, January 3, in Plattsmouth, Nebraska. The firefighters' water froze all over the building.
Michael Stanton walks between houses covered with ice in Scituate, Massachusetts, on January 3.
Pedestrians brave wind and snow as they cross New York City's Fifth Avenue on January 3.
Snow clings to the clothing and facial hair of Jerome Williams as he uses a snowblower in front of his home in Roosevelt, New York, on January 3.
Travelers wait in line January 3 at Chicago Midway International Airport.
Frost covers the windows at the Morning Glory natural food store in Brunswick, Maine, on January 3.
Snowplows clear snow from one of the runways at JFK Airport on January 3.
Surfers make their way through snow on New York's Rockaway Beach on January 3.
Passengers wait in line at a security checkpoint at JFK Airport on January 3.
Blowing snow swirls as a worker shovels a platform at a Haddonfield, New Jersey, train station on January 3.
A plane takes off January 3 as trucks plow snow at Newark Liberty International Airport in Newark, New Jersey.
A man walks down a snowy road along the shore in Scituate on January 3.
Postal worker Danny Kim clears snow and ice off the hood of his delivery truck January 3 in Bethesda, Maryland.
A couple walks across a snow-covered parking lot January 3 at Newark Liberty International Airport.
Tourists play in the snow at the base of the Washington Monument on January 3.
People play in Brooklyn's Prospect Park on January 3.
A man walks through the snow as the wind kicks it up January 3 in New York City.
Dante de Blasio, son of New York Mayor Bill de Blasio, shovels snow outside his home in Brooklyn on January 3.
Snow covers subway rails in New York City on January 3.
A man uses his snowblower to clear some paths in Mansfield, Connecticut, on January 3.
Workers clear snow off sidewalks on New York City's Fifth Avenue on January 3.
A man clears snow from a vehicle January 3 in Albany, New York.
A truck-mounted snowblower clears a section of road in Dedham, Massachusetts, on January 3.
A worker clears snow from a subway station in Queens, New York, on January 3. Public schools were closed Friday after up to 7 inches of snow fell in New York City.
A man shovels snow during the tail end of a snowstorm in Brooklyn on January 3.
A man rides an all-terrain vehicle through a Brooklyn street on January 3.
The Kunta Kinte-Alex Haley Memorial is covered in snow in Annapolis, Maryland, on January 3.
A man plows snow January 3 following an overnight snowstorm in Jersey City, New Jersey.
A child sleds down a hill in Chicago's Humboldt Park on Thursday, January 2.
Children make a snow pile in New York's Times Square on January 2.
Winds whip snow from the beach across Winthrop Shore Drive in Winthrop, Massachusetts.
Snow covers bikes along Leavitt Street in Chicago's Wicker Park on January 2.
With his dog in tow, a man skis across heavy snow in Humboldt Park on January 2.
Dallas Todd, 11, flies down a snowy hill at Lake Harbor Park in Norton Shores, Michigan, on January 2.
Snow covers cars in Chicago's Humboldt Park neighborhood on January 2.
A man drags a suitcase on a snowy street in downtown Boston on January 2.
A van is pulled from a ditch along Interstate 94 in Jackson, Michigan, on January 2.
A man walks his dog on the snow-covered Monon Trail in Carmel, Indiana, on January 2.
An airplane waits for passengers at O'Hare International Airport in Chicago on January 2.
A woman walks through snowy conditions in Albany on January 2.
Trucks clear snow off the roads in Torrington, Connecticut, on January 2.
Winter weather grips U.S.
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Photos: Winter weather grips U.S.
How frequently does this polar vortex distortion occur?
The upper-level winds that make up the polar vortex change in intensity from time to time. When those winds decrease significantly, it can allow the vortex to become distorted, and the result is a jet stream that plunges deep into southern latitudes, bringing the cold, dense Arctic air spilling down with it. This oscillation is known as the Arctic Oscillation and it can switch from a positive phase to negative phase a few times per year. This oscillation -- namely the negative phase where the polar winds are weaker -- tends to lead to major cold air outbreaks in one or more regions of the planet.
Where on Earth can this happen?
The polar vortex can lead to major cold air outbreaks in any portion of the Northern Hemisphere -- North America, Europe and Asia. This will lead to cold snaps in multiple locations, though not always.
How dangerous is a polar vortex distortion as compared to a tornado or hurricane?
Completely different type of systems. A cold air outbreak caused by the polar vortex is much more widespread and lasts longer than a single storm. With the widespread drop in temperature, however, you can see significant winter storms develop, especially when the cold air is initially advancing into a previously warm region -- much like the nor'easter this past week.
When was the last big one to hit a densely populated area?
Serious cold snaps happen several times a year, though in different regions of the world and with different severities. Last March saw a significant decrease in temperature as a result of the polar vortex pushing into much of Europe. Many locations experienced an Easter holiday that was much colder than their Christmas holiday. The United Kingdom, for instance, had its coldest March in 50 years.
If you get caught up in one, what should you do?
Again, it's not a "storm" that you get caught in. But when faced with significant cold temperatures, you should stay inside whenever possible, layer clothing if you must be outside, winterize your home and car, etc.
Is it a side effect of global warming and should we expect more events like this?
This is a hotly researched topic. In short, yes, it could be. It seems counterintuitive that global warming could cause significant cold snaps like this one, but some research shows that it could. We know that different types of extreme weather can result from the overall warming of the planet, melting of the Arctic Sea ice, etc. This includes extreme distortions of the jet stream, which can cause heat waves in summer and cold snaps in winter.
Parts of Western Europe have been battered by the worst storms for two decades during the past week -- are they related to the weather system in the United States?
They are related in a sense, but I wouldn't say "caused by." The storms in Europe are the result of a persistent pattern that has seen the jet stream parked near the United Kingdom and Ireland, which has brought a train of storm systems over the British Isles. This is the same jet stream, of course, that has plunged deep into the southern portions of the United States. So, the jet stream has been "stuck" in a position that is allowing cold Arctic air to spill into much of the United States and Canada. But it is in a position that is bringing warmer, moist air from the Atlantic over Northwest Europe, resulting in the stormy conditions.
cnn.com
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